1. Field of the Invention
This invention pertains generally to processing of audio signals, and more particularly to the processing and rendering over headphones of audio signals that change dynamically in response to head rotation.
2. Description of Related Art
U.S. Pat. No. 7,333,622 which is incorporated herein by reference in its entirety, describes a novel and effective method, denoted Motion Tracked Binaural (MTB), to capture and render over headphones the dynamic changes of binaural sound caused by the rotation of the listener's head. MTB uses a small number of microphones positioned on a head-sized spherical or cylindrical surface to achieve this goal. The basic problem that MTB solves is the interpolation of the signals obtained from adjacent microphones without requiring an impractical number of microphones. The MTB method exploits two important properties of human hearing:
(a) The interaural time difference or ITD is the dominant localization cue; and
(b) The auditory system is insensitive to ITD above about 1.5 kHz.
The spacing of microphones is determined by the highest frequency of the signals to be captured. The MTB method increases the spacing and thus reduces the number of microphones by first low-pass filtering the signals to remove spectral content above 1.5 kHz before interpolation. However, the high-frequency content is needed for good sound quality and must be restored. The MTB patent suggests several approximate ways to restore the high-frequency content. These methods proposed are completely general and apply to the capture and rendering of any soundfield. They do not depend on the knowledge of the number or locations of the sound sources. However, each specific method that combines low-pass filtering and high-frequency content restoration is an approximation, and each has its own audible artifacts.
Accordingly, an object of the present invention is continuous interpolation with no separation of low and high frequencies, i.e., to enable wide-band or full-bandwidth interpolation.